1. Field of the Invention
The present invention relates to an NMR cell system used for NMR measurements of supercritical fluids created under high-temperature and high-pressure conditions and to a high-pressure cell for use in an NMR spectrometer.
2. Description of the Related Art
When a fluid is placed at temperature and pressure conditions exceeding its critical point, the fluid becomes a supercritical fluid. The supercritical fluid has physical properties intermediate between those of gas phase and those of liquid phase. The physical properties (i.e., density, solubility, viscosity, dielectric constant, and self-diffusion coefficient) of the supercritical fluid are widely different from the physical properties in the ordinary liquid state. The supercritical fluid has a smaller density than in the liquid state, a higher solubility than in the liquid state, a lower viscosity than in the liquid state, a higher dielectric constant than in the liquid state, and a higher self-diffusion coefficient than in the liquid state. The physical properties can be varied continuously over a wide range by using the pressure or temperature as a parameter. Today, extraction methods and separation methods utilizing such physical properties intrinsic in supercritical fluids have been developed, and these methods find wide acceptance in food and chemical industries. Furthermore, there exists supercritical fluid chromatography (SFC) that is an analytical procedure making use of the properties of supercritical fluids.
Research into chemical reactions within supercritical fluids is a research field that has recently attracted attention, because a supercritical fluid has unique features that cannot be found in ordinary solutions. That is, it has a high solubility and a high diffusivity. In addition, the density can be varied continuously. If a supercritical fluid is regarded as a chemical reaction field, there is a possibility that a new field of reaction research different from chemical reactions within ordinary solutions is established. For example, in the case of water, at the subcritical state that is in immediate vicinity to the supercritical state, it is known that water exhibits strong acidity unlike normal water and dissolves various metals. It is found that water in the supercritical state dissolves polymeric organic substances, such as polyethylene terephthalate (PET), and substances that contaminate the environment, such as dioxin. It is expected that water in the supercritical state will be used as a chemical recycle technique for conserving the global environment.
Accordingly, there is an urgent demand for establishment of a technique for directly observing supercritical fluids by NMR. However, supercritical fluids exhibit various temperatures and pressures, depending on substances. FIG. 1 shows the distributions of the critical points of various chemical substances. As can be seen from the diagram of FIG. 1, where gaseous carbon dioxide in the supercritical state is observed, the temperature should be set to lower than 100xc2x0 C. and the pressure should be set to lower than 10 MPa. However, where water in the supercritical state should be observed, a higher temperature of around 400xc2x0 C. and a higher pressure of around 30 MPa are required. Accordingly, various NMR cell systems capable of coping with various experimental conditions and used for supercritical fluid measurements have been devised.
FIG. 2 shows a conventional NMR cell system used for supercritical fluid measurements. This system includes a cylindrical NMR cell 1 having a bottom. The cell is made of a ceramic such as zirconia or glass. The NMR cell 1 has an open end at which a cell holder 2 is mounted. The cell holder 2 is made of a nonmagnetic titanium alloy to prevent perturbation of the static magnetic field in the NMR spectrometer. The cell holder 2 is connected with a pressure transfer tube 4 made of stainless steel via a pipe connector portion 3 made of a titanium alloy and stainless steel.
In experiments using supercritical carbon dioxide, very high temperatures or very high pressures are not required. In this case, the NMR cell 1 is directly filled with liquefied carbon dioxide. Then, pressure is applied to the carbon dioxide from the pressure transfer tube 4. Under this condition, the cell 1 is inserted into an NMR probe (not shown). Thereafter, it is heated to a high temperature by a sample temperature control device (not shown), thus creating a supercritical fluid. Then, NMR measurements are performed. In experiments using alcohol or water, considerable high temperatures and pressures are required. In this case, facilities are also mounted outside the NMR cell 1 to apply a high pressure. This eliminates the pressure difference between the inside and the outside of the NMR cell 1. In this way, the NMR cell 1 is prevented from being destroyed due to pressure.
The conventional NMR cell system constructed in this way and fused for supercritical fluid measurements has the following problem. The bottom of the NMR cell is rapidly heated by the temperature control device. On the other hand, the top of the NMR cell is not heated as much by the temperature control device. Thus, a large temperature difference is created between these two portions. Therefore, if a supercritical fluid is created around the bottom of the NMR cell, the sample remains at the normal liquid state around the top of the NMR cell.
Since the density of a supercritical fluid is about one order of magnitude smaller than that of a normal fluid, if a high-temperature, low-density supercritical fluid phase is produced around the bottom of the NMR cell, a violent convection occurs with the low-temperature, high-density, normal liquid phase existing around the top of the NMR cell. The supercritical fluid phase and the normal liquid phase mix, thus greatly deteriorating the resolution of the NMR signal.
In addition to the problem of convection, there is another problem that new reactive gas or reactive liquid cannot be introduced because the inside of the NMR cell is in the normal pressure state or pressurized state. Furthermore, experiments on anaerobic systems cannot be conducted because it is impossible to remove the oxygen (air) from inside the NMR cell.
Under such high-temperature and high-pressure conditions, the high-pressure cell used for an NMR spectrometer is required to exhibit the following properties:
(1) During measurement, the homogeneity of the static magnetic field produced by the NMR spectrometer is not perturbed.
(2) During measurement, the radio frequency from the NMR spectrometer is not blocked.
(3) During measurement, the produced NMR background signal is low.
(4) A pressure of several 10 MPa can be applied to the sample.
(5) During NMR measurement, the pressure applied to the sample can be controlled.
(6) When a high pressure is applied to the sample, no leakage occurs from the joint of the cell.
(7) The sample can be easily loaded and exchanged. The cell can be easily cleaned.
(8) The temperature of the cell can be controlled under a high-pressure condition.
There is a strong demand for development of a high-pressure cell that is adapted for use in an NMR spectrometer and satisfies these conditions.
In view of the foregoing, it is an object of the present invention to provide an NMR cell system adapted for use in supercritical fluid measurements and having an NMR cell in which convection can be prevented and into which reactive gases and reactive liquids can be supplied smoothly, the NMR cell being further characterized in that the oxygen (air) in the cell can be removed before measurement.
It is another object of the present invention to provide a high-pressure cell which is for use in an NMR spectrometer, does not perturb the homogeneity of the static magnetic field produced in the NMR spectrometer during measurement, does not block the radio frequency generated from the NMR spectrometer, produces only low NMR background signals, permits a high pressure of several 10 MPa to be applied to the sample, permits the pressure applied to the sample to be controlled during NMR measurement, does not permit leakage from the joint of the cell during application of the high pressure to the sample, facilitates loading a sample, exchanging the cell, and cleaning the cell, and permits the sample temperature to be controlled under high-pressure conditions.
An NMR cell system for supercritical fluid measurements in accordance with the present invention has a cylindrical cell having a bottom and receiving a sample, a cell holder mounted at the open end of the cell, and an external pumping system. This NMR cell system is characterized in that a pipe is mounted to the cell holder to connect the external pumping system into the cell.
As another feature of this NMR cell system, the cell and the cell holder are made of a nonmagnetic material that withstands high temperatures, high pressures, and strong acidity.
As a further feature, the cell is made of a material that does not block a radio frequency and produces only low background signals in a given observed region of NMR.
As still another feature, the cell is made of a ceramic or sapphire.
As yet another feature, the cell holder is made of a titanium alloy.
As a still further feature, the aforementioned pumping system comprises a vacuum pump for evacuating the inside of the NMR cell system for supercritical fluid measurements and a pressure pump for introducing a sample into the cell after evacuation and applying pressure to the sample in the cell.
As a yet further feature, the above-described pumping system is equipped with a cleaning pump for cleaning the pipe of the NMR cell system for supercritical fluid measurements.
As an additional feature, a pressure system using a piston is mounted at one end of the cell to separate pressurized materials from the sample in the cell.
The present invention further provides an NMR cell system for supercritical fluid measurements, the system comprising a cell having a bottom, a cell holder mounted at the open end of the cell that receives a sample, and an external pumping system.
As one feature of this NMR cell system, a cell space-limiting means is mounted inside the cell to prevent convection of a supercritical fluid.
As another feature, the cell and the cell holder are made of a nonmagnetic material that withstands high temperatures, high pressures, and strong acidity.
As a further feature, the cell and the cell space-limiting means are made of a material that does not block a radio frequency and produces low background signals in a given observed region of NMR.
As yet another feature, the cell and the cell space-limiting means are made of a ceramic or sapphire.
Furthermore, the present invention provides a high-pressure cell for use in an NMR spectrometer, the high-pressure cell comprising a pressure-proof cell for receiving a sample to be investigated, a pressure transfer tube for transmitting pressure to the sample inside the pressure-proof cell, and a connector portion for coupling together the pressure-proof cell and the pressure transfer tube.
As one feature of this high-pressure cell for use in an NMR spectrometer, the pressure-proof cell is detachably mounted to the connector portion via a sealing member.
As yet another feature, the sealing member is an O-ring or a backup ring.
As an additional feature, the backup ring is made of a light metal (such as aluminum), copper, or a synthetic resin (such as Teflon).
In addition, the present invention provides a high-pressure cell for use in an NMR spectrometer, the high-pressure cell comprising a pressure-proof cell for receiving a sample to be investigated, a pressure transfer tube for transmitting pressure to the sample inside the cell, and a coupling portion for coupling together the pressure-proof cell and the pressure transfer tube. This high-pressure cell is characterized in that the pressure applied to the sample to be investigated can be varied while the pressure-proof cell is set in the measuring portion of the NMR spectrometer.
Further, the present invention provides a high-pressure cell for use in an NMR spectrometer, the high-pressure cell comprising a pressure-proof cell for receiving a sample to be investigated, a pressure transfer tube for transmitting pressure to the sample inside the pressure-proof cell, and a coupling portion for coupling together the pressure-proof cell and the pressure transfer tube. This high-pressure cell is characterized in that the temperature of the sample to be investigated can be varied while the pressure-proof cell is set in the measuring portion of the NMR spectrometer.
Other objects and features of the invention will appear in the course of the description thereof, which follows.